1. Field of the Invention
The present invention relates to a release process film used by attaching to a flexible printed circuit board. More specifically, the present invention relates to a release process film which is attached to the surface of a flexible printed circuit board, effectively suppresses contamination of the surface of a printed circuit board with solvents and foreign substances and formation of flaws on the surface, leaves little adhesive residue on the adherend after being peeled off and suppresses curling of the adherend in the process for producing electric and electronic instruments by mounting electric and electronic members to the flexible printed circuit board.
2. Description of Related Art
A printed circuit board is a circuit board having a circuit pattern of a conductive material which is necessary for connecting electric and electronic members mounted on the board and formed by printing on the surface of or on the surface and at the inside of an insulating substrate. The printed circuit board has the function of electrically connecting electric and electronic members mounted in electric and electronic instruments through the circuit.
From the standpoint of the form, the printed circuit board can be classified into rigid printed circuit boards and flexible printed circuit boards. From the standpoint of the structure, the printed circuit board can be classified into single layer, multi-layer, i.e., having two or more layers, and flexible printed circuit boards. From the standpoint of the application, most of the printed circuit boards for instruments of household applications are single layer printed circuit boards, and most of the printed circuit boards for instruments for industrial applications are multi-layer printed circuit boards.
Among the above printed circuit boards, the flexible printed circuit board is obtained by forming a circuit pattern of a conductive material on an insulating substrate having excellent flexibility and is indispensable for steric high density mounting in a narrow space having a complicated structure at the inside of electronic instruments. The flexible printed circuit board is one of members of electronic instruments which are most useful for satisfying the recent requirements for the electronic instruments to decrease the size and the weight and increase the density and the accuracy, and the demand for the flexible printed circuit board is rapidly increasing.
Since flexibility is required for the above flexible printed circuit board, a plastic sheet exhibiting excellent heat resistance and electric insulation such as a polyimide sheet and a polyphenylene sulfide sheet is used as the insulating substrate, and a circuit pattern is formed on the surface of or on the surface and at the inside of the insulating substrate with a metal material such as copper. The important properties required for the flexible printed circuit board are, for example, dimensional accuracy, small degrees of curling and twisting, heat resistance, peeling strength, bending strength, volume resistivity, surface resistance of the adhesive, chemical resistance, bending strength under heating, holding power and properties suitable for working as a printed circuit board.
To the above flexible printed circuit board, in general, a release process film is attached in advance before electric and electronic members are mounted so that contamination with solvents and foreign substances and formation of flaws are suppressed in the process for producing the electric and electronic instruments by mounting the electric and electronic members. In the process film, an adhesive layer which can be released is disposed on one face of the base material film. After the process film is attached to the flexible printed circuit board, the flexible printed circuit is treated in various processes such as die cutting, dipping into a solvent and heat pressing, and the process film is thereafter peeled off the flexible printed circuit board. Therefore, it is important that lifting or peeling of the process film does not take place in the above processes and the process film exhibits the release property such that no adhesive residue is left on the adherend and curling of the adherend does not take place after the process film is peeled off.
As the release adhesive film, for example, heat resistant release adhesive films having an adhesive layer formed by using an acrylic adhesive of the crosslinking type containing an aliphatic polyisocyanate as the crosslinking agent are disclosed (for example, Japanese Patent Application Laid-Open No. 2000-44896). In the acrylic adhesive of the crosslinking type, in general, a material containing 2-ethylhexyl acrylate unit as the main component is frequently used as the resin component so that the excellent release property is obtained. However, the acrylic adhesive of the crosslinking type using a resin composition containing 2-ethylhexyl acrylate as the main component described above has a drawback in that, when an adhesive film using the adhesive is attached to a flexible printed circuit board as the process film, shear occasionally takes place between the adhesive film and the flexible printed circuit board during the heat pressing operation to cause disorder in the dimension and, as the result, it becomes difficult that the accurate electric circuit is obtained.